The subject of the invention is a method for the production of enantiomeric isoflavanes from isoflavones, and especially for producing R- or S-equol from daidzein. The method is characterized by a first reduction step and covalent attachment of a chiral compound, thereby obtaining a chiral intermediate.
Isoflavones, also referred to as isoflavonoids, are a class of organic compounds which often occur naturally. Isoflavones are compounds from plants, which, amongst others, play a role in the plants' defense against pathogens. Many isoflavones act as phytoestrogens in mammals. Some are antioxidants because of their ability to trap singlet oxygen. Some well-known isoflavones are daidzein, found as a glucoside of daidzin in soy flour, genistein from soy beans and red clover, prunetin from the bark of plum trees, biochanin A from chickpeas and clover, orobol, santal from sandle wood, red wood and other woods and pratensein from fresh red clover. Some isoflavones, in particular soy isoflavones, when studied in populations eating soy protein, have indicated that there is a lower incidence of breast cancer and other common cancers because of its role in influencing sex hormone metabolism and biological activity through intracellular enzymes, protein synthesis, growth factor actions, malignant cell proliferations, differentiation and angiogenesis. The isoflavone daidzein [4′,7-dihydroxyisoflavone; 7-hydroxy-3-(4-hydroxyphenyl)-4H-1-benzopyran-4-one] is found in soy. It is a part of many foods and dietary supplements. It is easily available in relatively large amounts from soy.
Isoflavanes are synthesized in physiological pathways from isoflavones by selective reduction of the basic structure. The isoflavane equol [4′,7-dihydroxyisoflavane; 3-(4-hydroxyphenyl)-7-chromanol] is produced in the intestinal flora after consumption of daidzein. The reaction is thought to be mediated by bacteria. Equol is thus a part of the group of secondary plant metabolites. After consumption of food which is rich in daidzein, equol is detectable in blood and urine. Equol has a mild estrogenic activity (0.1% of the activity of steroid-estrogens) and can bind to the estrogen receptors ERα and ERβ. Only about a third (Caucasian population) up to half (Japanese population) of humans can produce equol from daidzein. In humans who are capable of producing equol (“equol producers”), the cholesterol reducing and anti-inflammatory effect of a soy-rich diet is more pronounced when compared to humans, which are not capable of producing equol. In contrast to daidzein, naturally formed equol is chiral due to an asymmetric C3 atom. Various physiological activities have been attributed to S-equol as well as to R-equol. For S-equol, anti-proliferative effects were demonstrated in studies, for instance with respect to tissue changes in the breast which may occur in females during menopause. Equol inhibits DHT production in males due to interactions with the 5α reductase. It is assumed that DHT is a cause in the forming of prostate cancer in males.
The advantageous effects of isoflavanes and phytoestrogens such as equol are usually observed, if the compounds are consumed over an extended time period in significant amounts. Since the equol precursor daidzein is found in comparatively large amounts in soy, equol could be administered as a dietary supplement (food supplement, nutritional supplement). It would thus be highly desirable that equol is available by a simple production method, in large amounts and at low costs. Since equol is produced from daidzein in the intestine, it cannot be isolated in significant amounts from plants or other natural products. Thus, it can only be obtained in significant amounts by artificial synthesis.
Various publications disclose microbiological methods for producing equol. In this respect, EP 1 025 850 discloses a composition in which equol is produced microbiologically from soy isoflavones. However, microbiological production processes have various disadvantages, because microorganisms tend to change and thus do not yield uniform products. The production process thus has to be supervised continually, also to avoid contaminations. Further, the microorganisms have to be eliminated and removed from the composition after the production is completed.
Thus, there have been many efforts for producing equol by means of organic synthesis. For example, Muthyala et al. (Bioorg. Med. Chem., 2004, 12, p 1559-1567) describe a method for the production of equol from daidzein, which comprises a reduction of the 2,3-double bond and the keto group in the presence of a palladium hydroxide-catalyst of formula Pd(OH)2. A racemate is obtained, which is separated into R-equol and S-equol by chromatography (HPLC). However, separation of racemates is generally difficult at industrial scale and chiral media for chromatographic separation are rather expensive.
Heemstra et al. (Organic Letters, 2006, 8, p 5441-5443) disclose an asymmetric synthesis of the chromane-ring by means of Evans-alkylation and intramolecular etherification according to Buchwald.
Patent application WO 2007/016423 A2 discloses a method for producing equol, which comprises reducing the 2,3-double bond and the 4-keto group of a substrate, elimination of the 4-OH-group under formation of a 3,4-double bond, synthesis of a specific iridium-catalyst and subsequent enantioselective reduction of the 3,4-double bond with the iridium catalyst.
Published patent application WO 2010/018199 A1 discloses a multiple-step process for producing isoflavanes from isoflavones, in which the 4-keto group is reduced in a first step in an enantioselective manner to the 4-hydroxy compound, whilst the C2,C3-double bond is maintained. In further steps, a protective group is attached to the 4-hydroxy group and the C2,C3-double bond is reduced. The process avoids precious metals and separation of enantiomers.
There is an ongoing need for alternative or improved synthesis methods, which increase the enantiomeric yield and render the overall process simpler and more economical. The known methods for producing equol are still relatively complicated, require multiple steps, or special and expensive reagents, or the yield is not high. Some processes require complex chiral catalysts and precious metals and multiple process steps. Some methods yield a racemate and require final separation into enantiomers, which requires multiple process steps and is complicated, especially at industrial scale. A simple and cost-efficient production, which would allow supplying large parts of the population with an adequate daily dose, is presently not available. Thus, in spite of all attempts for providing efficient methods for producing equol, the market price for R- or S-equol is still extremely high, presently more than 200 EUR per mg. Thus, R- or S-equol is presently still not commercially available in amounts for meeting therapeutic demands. Other isoflavanes, which also may have beneficial properties, are also not available.
Problem Underlying the Invention
The problem underlying the present invention is to provide a simple and cost-efficient method for the production of chiral isoflavanes, especially R- or S-equol. The method shall be applicable for obtaining the isoflavane at high enantiomeric and total yield. The method shall only comprise few reaction steps, which shall be simple. The use of complicated chemicals, such as chiral metal catalysts, shall be avoided. The use of toxic chemicals, which are not acceptable in pharmaceutical preparations, and thus have to be removed carefully in subsequent steps, shall be avoided. For example, reactive metals, like ruthenium or iridium, shall be avoided. The method shall enable synthesis of various isoflavanes, equol and derivatives thereof in a simple and convenient manner. Overall, the method shall provide for economical production of large amounts of isoflavanes.